A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a scanner, or scanning lithographic apparatus, the apparatus includes a substrate or wafer table that is constructed to hold a substrate, e.g. a resist-coated wafer. During exposure of the substrate, the substrate table is moved in a scan direction (usually denoted as Y-direction) with respect to a projection system, such as a lens assembly that is configured to project a pattern imparted to a radiation beam by a patterning device such as a mask onto a target portion on the substrate. During this scanning motion, or scan movement, the speed of the substrate in the scan direction is kept constant. The speed of the substrate during the scan movement is not arbitrary. On the one hand the speed of the substrate, i.e. the speed of the substrate table, is limited due to the mechanical properties of the lithographic apparatus for example, on the other hand it is desirable to optimize the speed of the substrate during scanning to expose the substrate as fast as possible.
The usual way of determining this optimized scan speed is to minimize the time required to cover the distance to be traveled during the scan movement, i.e. the distance in the scan direction. This optimization of the scan speed takes into account this distance, which is equal to the size of the target portion in the scan direction including the slit size, i.e. the distance between two neighboring target portions. Further, the acceleration of the substrate in the scan direction is taken into account in the determination of the optimized scan speed. This way of determining the optimized scan speed has been used in lithography for many years.